I guess my RO wasn't kidding when he said, "Stat". Lumbar MRI ordered this morning, completed this afternoon, results posted this evening.
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Impression:

Osseous metastatic disease replaces the marrow of the L5 vertebral body. More diffuse marrow heterogeneity is noted which is likely related to diffuse marrow infiltration.

No significant spinal canal stenosis is noted in the lumbar region. Please see additional comments above.

Procedure: MR LUMBAR SPINE W WO CONTRAST

History: 70-year-old patient with L5 metastatic disease.

There are 5 lumbar type vertebral bodies noted.

Findings:

Diffuse marrow replacement is identified in the L5 vertebral body. This is consistent with osseous metastatic disease. No significant epidural component is demonstrated. Diffuse heterogeneous marrow signal suspicious for marrow infiltration is noted throughout the lumbar spine and sacral region.

No compression deformities are noted. The distal spinal cord and conus is of normal signal intensity.

Broad-based disc bulges are present at all levels in the lumbar region. A superimposed central disc protrusion is noted at L2-L3. A superimposed LEFT paracentral disc protrusion is noted at L4-L5.

Moderate bilateral foraminal narrowing is noted at L5-S1.
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I'll reserve any speculation until I hear from RO other than to say, this report is no bueno. Apparently, there's a reason my back hurts.